Recently, there has been an increase in the number of vehicles equipped with a pedestrian protection device such as a deployable hood (i.e., pop up hood), a hood airbag, or the like. Such a pedestrian protection device is designed to be activated in a collision of a vehicle with a pedestrian. As disclosed in, for example, US 2007/0181359 corresponding to WO 2005/098384 or US 2006/0087417 corresponding to JP-A-2006-117157, a collision detection sensor has been proposed that detects a collision of a vehicle bumper with a pedestrian. The collision detection sensor includes a pressure sensor for detecting pressure in a chamber room. The chamber room is defined by a chamber member located between the bumper and a bumper reinforcement of a vehicle. When the bumper collides with the pedestrian, the chamber member is deformed so that the pressure in the chamber room can change. Based on the pressure change in the chamber room, the collision detection sensor determines whether the bumper collides with the pedestrian.
The pressure change may occur as a result of the deformation of the chamber member, even when the bumper collides with an object other than a pedestrian. For example, the chamber member may be deformed, when the bumper collides with a lightweight object such as a construction cone. However, there is no need to activate a pedestrian protection device in the event of such a light collision.